A metal material is often provided with an overlying coating, for example, for improving its corrosion resistance or some design purpose. In most cases, a chemical conversion film is present between the coating formed by the coating and the metal material, and this chemical conversion film greatly improves the corrosion resistance and an adhesion properties of the coating. The chemical conversion film is formed on the surface of the metal material by the process called “chemical conversion” which is accomplished by bringing the surface of the metal material with a chemical agent called “chemical conversion solution”. Examples of the chemical conversion known in the art that are used for imparting the metal material with corrosion resistance and coating adhesion include chromate treatment, zinc phosphate treatment, and zirconium-based treatment.
Among these, in the case of chromate treatment, hexavalent chromium is present in both the solution used for the chemical conversion and in the resulting chemical conversion film. As a consequence, use of the chromate treatment is limited by environmental reasons. In addition, use of the chromate treatment was difficult for structure which was partly formed from an iron or steel material since coating weight was not sufficient when used for an iron-based material while it was effective for use with a zinc-plated material or an aluminum alloy material.
Zinc phosphate treatment is effective not only for the zinc-based plated materials and aluminum alloy materials but also for iron and steel materials, and this treatment is also quite adequate as an undercoat for various coatings, and in particular, in the case of cationic electrodeposition. However, the solution used in the zinc phosphate treatment contains phosphorus which is a eutrophic element as well as nickel which has the risk of carcinogenicity. Treatment with zinc phosphate is also associated with the generation of industrial waste called “sludge” as the byproduct, and the trend has been against the use of this treatment for environmental concern.
In contrast, zirconium-based chemical conversion is capable of forming the required amount of coating on various materials with improved corrosion resistance and coating adhesion, and such effects are realized with reduced damage on the environment.
As an example of such zirconium-based chemical conversion, Patent Literature 1 discloses a chemical conversion agent comprising at least one member selected from the group consisting of zirconium, titanium, and hafnium; fluorine; and an agent for imparting the adhesion and the corrosion resistance, in which the agent for imparting adhesion and corrosion resistance is at least one member selected from the group consisting of an ion of a metal such as zinc, an alkaline earth metal ion, an ion of a metal in Group III in the periodic table, copper ion, and a silicon-containing compound.
Patent Literature 2 discloses a chemical conversion agent comprising at least one member selected from the group consisting of zirconium, titanium, and hafnium; fluorine; an agent for imparting adhesion, and a chemical conversion accelerator, in which the agent for imparting adhesion is at least one member selected from the group consisting of an ion of a metal such as zinc, an alkaline earth metal ion, an ion of a metal in Group III in the periodic table, copper ion, a silicon-containing compound, a water-soluble resin, a water-soluble epoxy compound, a silane coupling agent, and/or hydrolysate thereof. The unique feature of Patent Literature 2 is the chemical conversion promoter.
Both Patent Literature 1 and Patent Literature 2 further disclose a surface treated metal having a chemical conversion film formed by using such chemical conversion agent on its surface. As the metal substrate of the surface treated metal, Patent Literature 1 and Patent Literature 2 disclose an iron-based substrate, an aluminum-based substrate, and a zinc-based substrate, and with regard to the shape of the metal substrate, they only disclose simple plate shape. Patent Literature 1 and Patent Literature 2 also describe that the coating that may be deposited on the metal substrate having the chemical conversion film formed by using the chemical conversion agent is not particularly limited, and exemplary coatings include cationic electrodeposition and powder coating.
Such zirconium-based chemical conversion is capable of forming the required amount of chemical conversion film on various metal materials to impart the metal material with corrosion resistance, and the film can be formed with reduced damage to the environment. In the case of the zirconium-based chemical conversion, coating adhesion after the cationic electrodeposition is also improved.